The field of wireless communications is expanding at a phenomenal rate, as more radio spectrum becomes available for commercial use and as cellular phones become commonplace. In addition, there is currently an evolution from analog communications to digital communications. In digital communications, speech is represented by a series of bits which are modulated and transmitted from a base station to a phone. The phone demodulates the received waveform to recover the bits, which are then converted back into speech. There is also a growing demand for data services, such as e-mail and Internet access, which require digital communications.
There are many types of digital communications systems. Traditionally, frequency division-multiple-access (FDMA) is used to divide the spectrum up into a plurality of radio channels corresponding to different carrier frequencies. These carriers may be further divided into time slots, a technique referred to as time-division-multiple-access (TDMA), as is done in the D-AMPS, PDC, and GSM digital cellular systems.
If the radio channel is wide enough, multiple users can use the same channel using spread spectrum (SS) techniques and code-division-multiple-access (CDMA). IS-95 and JSTD-008 are examples of CDMA standards. With direct sequence spread spectrum (DS-SS), information symbols are represented by sequences of symbols referred to as chips. This spreads the information symbols in the frequency band. At the receiver, correlations to the chip sequences are used to recover the information symbols. Spreading allows the system to operate at a low chip signal-to-noise ratio (SNR). If thermal noise is not too great, then noise from other users is tolerable and multiple user signals can occupy the same bandwidth at the same time.
The radio signal is reflected and scattered off of various objects, giving rise to multipath propagation. As a result, multiple images of the signal arrive at the receive antenna. When these images have roughly the same delay, relative to the chip period, then they give rise to fading. Fading occurs because the images add sometimes constructively, and sometimes destructively. When these images arrive with different delays relative to the chip period, they can be viewed as echoes of the signal and are often referred to as “resolvable multipaths”, “rays”, or simply “multipaths”.
To communicate efficiently and reliably, the receiver should exploit the multipath fading channel by collecting signal energy from the different multipaths. This is achieved by employing a RAKE receiver, which individually detects each echo signal using a correlation method, corrects for different time delays, and combines the detected echo signals coherently. The RAKE receiver includes a number of processing elements or “fingers”. The receiver must estimate the delays of the multipaths and assign a finger to each delay. The finger then despreads that signal image. The finger outputs are RAKE combined by weighting them and adding them together.
For mobile communications, the phone or the environment moves, so that multipath delays change over time. To maintain performance, the delay estimation procedure must be able to track the multipath delays. Traditional approaches to delay tracking are the early/late gate and tau-dither approaches. With these approaches, the signal energy is measured slightly before and slightly after the estimated delay. When the estimated delay is correct, then the early and late measurements should be approximately equal, as the chip pulse waveform falls off symmetrically about its peak. When an imbalance is detected, the delay estimate is adjusted to restore balance.
Specifically, an early-late gate (ELG) for each RAKE receiver finger operates as follows. Each ELG is provided with an initial estimate of the delay, τest, of one of the channel paths. The initial delay estimates for each path are typically within half a pseudo-noise (PN) code chip from the exact delay, τexact, of that path, i.e., |τexact−τest|≦0.5Tc. The ELG makes two correlations between the local PN code and the received DS-SS signal. One correlation uses a delay τest+δ, i.e., early correlation, while the other uses a delay, τest−δ, i.e., late correlation. The value of δ is typically 0.5Tc, or slightly less. FIG. 1 shows an example of the correlation function of the received SS signal versus τ. The early and late correlations are given by C(τest+δ) and C(τest−δ), respectively.
The results of the early and late correlations, C(τest+δ) and C(τest−δ), are compared and the initial estimation τest is updated. For example, in FIG. 1 the early correlation result C(τest+δ) is larger than the late correlation result C(τest−δ). Hence, the initial assigned delay τest is increased by a small value ε<δ, and the new estimate becomes τest/new=τest/old+ε. The process is continuously repeated. Eventually, the estimated delay τest converges to the exact channel path delay τexact after a number of iterations. In this case, i.e., when rest τest=τexact the results of the early and late correlations become equal and τest is not changed any more. The ELG relies on the fact that the correlation function resulting from the correlation of the transmitted SS code and the local code is symmetrical. Hence, when τest=τexact the early and late correlation results at τest+δ and τest−δ are equal. This is the case when the channel, shown by block 304 in FIG. 3, is a single path. FIG. 1 shows an example of the correlation function in this case. However, when the channel shown in block 304 of FIG. 3 is a multipath fading channel, the correlation function is no longer symmetrical.
For example, FIG. 2 shows the correlation function in a two path fading channel. The total correlation function due to the combination of two paths is given by the dashed line. If the channel has two paths at delays τ1,exact and τ2,exact, where τ1,exact<τ2,exact, then the ELG assigned to track the second path will have different correlation values at the early correlation at τ2,est+δ and the late correlation at τ2,est−δ even if τ2,est=τ2,exact. The late correlation is more influenced by interference from the other path than the early correlation. Hence, as shown in FIG. 2, even if τ2,est=τ2,exact the early and late correlations are not equal and τ2,est will be increased or decreased by ε until the early and late correlations are equal. When the early and late correlations are equal, τ2,est≠τ2,exact. Hence, in a multipath fading channel the conventional ELG is not able to track the multipath delays accurately. This shortcoming is reported in “Frequency Selective Propogation Effects on Spread Spectrum Receiver Tracking” by Robert L. Bogusch, Fred W. Guigliano, Dennis L. Knepp, and Allen H. Michelet, Proceedings of the the IEEE, Vol. 69, No. 7, July 1981, but no solution is provided.
An alternative to the ELG approach is given in Baier et al., “Design study for a CDMA-based third-generation mobile radio system”, IEEE Journal on Selected Areas in Communications, vol. 12, pp. 733-743, May 1994. In this paper, the baseband signal is sampled twice per chip period. Delays are estimated on a frame by frame basis. Data are despread using data-dependent despreading sequences and a matched filter (sliding correlator). This provides a sequence of correlation values corresponding to delays spaced Tc/2 apart, where Tc is the chip period. The magnitude squared of this sequence is taken and then averaged with other measurements, providing an estimated delay power spectrum. This delay spectrum is then searched for the strongest rays.
One concern with this approach is that, when the chip pulse shape is fairly broad, the approach will find several peaks next to each other, which really correspond to only one ray. This problem would become more pronounced should more samples per chip be taken.
Another concern, which was also a concern for the ELG approach, is the interpath interference. This can cause peaks to be selected that do not correspond to the actual delays.